Polytetraflouroethylene (“PTFE”) is a chemically inert polymer with a wide service temperature of −450° F.-500° F. PTFE is the material of choice for such industrial products as tubing, o-rings, diaphragms, valve seats, gaskets and seals. However, PTFE in its original pure form is known for high deformation or creep under load. Flange gaskets made of pure PTFE will flow or creep under the stress of bolt load, resulting in a loss in flange tightness and subsequent leakage over time. Lip seals made of pure PTFE will also deform under pressure resulting in leakage. To overcome the high creep problem fillers are incorporated in the matrix of PTFE. Filled PTFE materials such as this were first commercialized by Garlock Sealing Technologies in 1968 and marketed as GYLON gasket materials. GYLON Styles are reinforced with various filler materials, including glass micro-spheres, silica, barium sulfate and graphite at different percent filler levels. Filler material lowers creep and improves functional performance of the PTFE matrix. However, the filler material can result in porosity of the PTFE matrix, cause discoloration or contamination.
Referring to FIGS. 1 and 10, scanning electron microscope (“SEM”) images of flake graphite powder are shown. Currently, graphite-filled PTFE is used as a gasket material in monomer service applications. For example, graphite-filled PTFE is commercially available and sold by Garlock Sealing Technologies as GYLON Style 3530 sheets, gaskets, and lip seals. GYLON Style 3530 was introduced in about 1982.
Graphite filled PTFE, which is black in color, has an average particle size of about 5 to 100 μm and a agglomerated filler particle shape of a hexagonal platelets. Other properties of graphite filled PTFE are as follows. The average tensile strength (ASTM D1708) of graphite-filled PTFE is about 4000 psi. The compressibility (ASTM F36) is about 12 percent. The recovery (ASTM F36) is about 54 percent. The creep relaxation (ASTM F36) is about 56 percent. The sealability measured as gas permeability (DIN 3535) is about 0.011 cc/min. The weight change after a 16 hour soak in Magnaflux SKL-WP at 1 ATM is 0.05 percent. The weight change after a 16 hour soak in Magnaflux SKL-WP at 30 ATM is about 0.07 percent. The Magnaflux SKL-WP soak is a penetrability test that measures how much of a dye penetrates the sample and is measured by weight change of the sample as well as visual inspection.
Graphite filled PTFE is perceived as contaminating and may not be acceptable in color-sensitive environments such as biological, pharmaceutical applications, or chemical process applications. Particles of graphite from the graphite filled PTFE could become free and contaminate the media. Adding a dye cannot mask the black color of graphite. Graphite filled PTFE also has a significant number of voids. In some conventional processes, a gasket of graphite filled PTFE can be structurally weakened by the media. The monomer media can polymerize in the gasket, expand and cause the gasket to split.